N, K  = map(int, input().split())

if N % 2 != 0:
    if 0 < K <= N and (N+1)//2 != K:
        print(N-2)
    elif 0 < K <= N and (N+1)//2 == K:
        print(N-1)
    else:
        print(0)

else:
    if 0 < K <= N:
        print(N-2)
    else:
        print(0)